It is desirous to produce rounded toner particles, regardless of manufacturing origin, for a variety of reasons. The smoother surface affords fewer points of contact between toner and other surfaces in general, thus facilitating the removal of toner therefrom. As the trend to smaller and smaller toner sizes has occurred, the fundamental limitation of non-rounded toner is that it is difficult to convey in all regards. This is particularly critical in the development and transfer steps, from which much of the print quality is derived.
While chemically prepared toner (toner prepared in situ in liquid) offers the advantage of a rounder or smoother toner surface than conventional toner, a degree of smoothness and or circularity is not always necessary to capture the benefits of development and transfer. Smaller size in itself advances the level of print quality, all other considerations fixed. However, the complication of surface adhesion dramatically limits the efficiencies of development and transfer, thus offsetting the perceived advantage of reduced size. The introduction of the rounded surface overcomes these limitations, and enables both development and transfer to be optimized without reservation. Absent this shape advantage, transfer of toner less than 8 microns in size is particularly handicapped due to the ionization of air (field breakdown) which nearly always occurs prior to the attainment of optimal transfer.
The open and patent literature is full of references for the rounding of fractured toners, and there are a variety of ways in which this has been done. There is mechanical milling, actually just modified jet milling, in which the particles are rounded during the milling operation. The toner particles may be suspended in a hot air stream and rounded in that fashion. Other rounding devices utilize air bearings, wherein an air stream is forced through the device continuously in order to prevent particles from interfering with the motion of the stirring assembly and to create a fluidized bed; this requires an outlet to atmosphere though which the air flow may be vented.
The CYCLOMIX commercial mixing device (product of Hosokawa) creates a recirculating air stream surrounded by a heating/cooling jacket. This invention employs in its current implementation such a mixing device. Rounding occurs during repeated collisions with other particles and with the blade/vessel wall.
Since such mixing is very vigorous, particulate silica is included with the toner to prevent agglomeration of the toner during the mixing. This results in significant particulate silica being embedded in the rounded toner particles which further results in increased viscosity of the toner, thereby destroying adequate toner function for standard toner.